Examination of a lens 'native' plasma membrane fraction and its associated crystallins

Thermal stress induced aggregation of aquaporin 0 (AQP0) and protection by α-crystallin via its chaperone function

Aquaporin 0 (AQP0) formerly known as membrane intrinsic protein (MIP), is expressed exclusively in the lens during terminal differentiation of fiber cells. AQP0 plays an important role not only in the regulation of water content but also in cell-to-cell adhesion of the lens fiber cells. We have investigated the thermal stress-induced structural alterations of detergent (octyl glucoside)-solubilized calf lens AQP0. The results show an increase in the amount of AQP0 that aggregated as the temperature increased from 40°C to 65°C. α-Crystallin, molecular chaperone abundantly present in the eye lens, completely prevented the AQP0 aggregation at a 1∶1 (weight/weight) ratio. Since α-crystallin consists of two gene products namely αA- and αB-crystallins, we have tested the recombinant proteins on their ability to prevent thermal-stress induced AQP0 aggregation. In contrast to the general observation made with other target proteins, αA-crystallin exhibited better chaperone-like activity towards AQP0 compared to αB-crystallin. Neither post-translational modifications (glycation) nor C-terminus truncation of AQP0 have any appreciable effect on its thermal aggregation properties. α-Crystallin offers similar protection against thermal aggregation as in the case of the unmodified AQP0, suggesting that αcrystallin may bind to either intracellular loops or other residues of AQP0 that become exposed during thermal stress. Far-UV circular dichroism studies indicated a loss of αhelical structures when AQP0 was subjected to temperatures above 45°C, and the presence of α-crystallin stabilized these secondary structures. We report here, for the first time, that α-crystallin protects AQP0 from thermal aggregation. Since stress-induced structural perturbations of AQP0 may affect the integrity of the lens, presence of the molecular chaperone, α-crystallin (particularly αA-crystallin) in close proximity to the lens membrane is physiologically relevant.

Interaction of mammalian Hsp22 with lipid membranes

Hsp22/HspB8 is a member of the small heat-shock protein family, whose function is not yet completely understood. Our immunolocalization studies in a human neuroblastoma cell line, SK-N-SH, using confocal microscopy show that a significant fraction of Hsp22 is localized to the plasma membrane. We therefore investigated its interactions with lipid vesicles in vitro. Intrinsic tryptophan fluorescence is quenched in the presence of lipid vesicles derived from either bovine brain lipid extract or purified lipids. Time-resolved fluorescence studies show a decrease in the lifetimes of the tryptophan residues. Both of these results indicate burial of some tryptophan residues of Hsp22 upon interaction with lipid vesicles. Membrane interactions also lead to increase in fluorescence polarization of Hsp22. Gel-filtration chromatography shows that Hsp22 binds stably with lipid vesicles; the extent of binding depends on the nature of the lipid. Hsp22 binds more strongly to vesicles made of lipids containing a phosphatidic acid, phosphatidylinositol or phosphatidylserine headgroup (known to be present in the inner leaflet of plasma membrane) compared with lipid vesicles made of a phosphatidylcholine head-group alone. Far-UV CD spectra reveal conformational changes upon binding to the lipid vesicles or in membrane-mimetic solvent, trifluoroethanol. Thus our fluorescence, CD and gel-filtration studies show that Hsp22 interacts with membrane and this interaction leads to stable binding and conformational changes. The present study therefore clearly demonstrates that Hsp22 exhibits potential membrane interaction that may play an important role in its cellular functions.

Sphingolipids in human lens membranes: an update on their composition and possible biological implications

The unique nature of the most abundant phospholipids in human lens membranes remained overlooked until the 1990s when it was possible to discern dihydrosphingomyelins (DHSMs) from the more common sphingomyelins (SMs). Unlike in other mammalian membranes, DHSMs comprise nearly half of the phospholipids in adult human lenses. Compared to SMs with a trans double bond between carbons 4 and 5 of the sphingoid backbone, the absence of this unsaturation site in DHSMs allows the participation of the OH group on C3 in intermolecular H-bonds and leads to stronger interlipid interactions with both neighboring DHSMs and cholesterol. Phospholipid compositional changes with age and lens region observed in mammals with various life spans and lens growth rates, suggest that the highest levels of DHSMs along with the lowest amounts of phosphatidylcholines and SMs are found in lenses with the lowest growth rate, namely human lenses. The participation of phospholipid metabolites in the control of mitosis and elongation of lens cells is plausible and deserves investigation.

Lens membrane fraction associated intermediate filaments of different aged rats

PURPOSE

To describe the intermediate filament proteins vimentin, filensin and phakinin associated with different fractions isolated from neonatal, 10 day old and 20 day old rat lenses.

METHODS

Fractions were isolated by differential and density gradient centrifugation of lens homogenates from neonatal, 10 day old and 20 day old rats. Aliquots of the 8 M urea soluble proteins of each fraction were separated by SDS PAGE, transferred to PVDF membranes, the membranes were probed with antibodies to vimentin, filensin or phakinin, and analyzed by computer.

RESULTS

Over the 20 day growth period, the water soluble fraction increased and the most abundant membrane fraction was characterized by a significant increase in its urea insoluble protein and a significant decrease in its urea soluble protein. There were no significant quantitative changes in any of the other fractions. The concentration of each intermediate filament protein was greatest in the cytoskeletal fraction and over the 20 day period, the amount of vimentin associated with this fraction dramatically decreased, and the amounts of filensin and phakinin dramatically increased. Among the membrane fractions, the greatest concentration of each intermediate filament protein was found in the non sedimenting membrane fraction (NSMF) which was the least abundant fraction recovered. Filensin and phakinin associated with the other three major membrane fractions increased over the 20 day growth period, but the level of vimentin did not significantly change.

CONCLUSIONS

The NSMF may represent a domain of the lens plasma membrane particularly important in interaction between plasma membrane and cytoskeleton and as the membrane-cytoskeleton protein architecture of rat lens changes over the first 20 days of life, the changes are readily detected in the different membrane fractions.

Characterization of alpha-crystallin-plasma membrane binding

Alpha-crystallin, a large lenticular protein complex made up of two related subunits (alphaA- and alphaB-crystallin), is known to associate increasingly with fiber cell plasma membranes with age and/or the onset of cataract. To understand better the binding mechanism, we developed a sensitive membrane binding assay using lens plasma membranes and recombinant human alphaA- and alphaB-crystallins conjugated to a small fluorescent tag (Alexa350). Both alphaA and alphaB homopolymer complexes, as well as a reconstituted 3:1 heteromeric complex, bind to lens membranes in a specific, saturable, and partially irreversible manner that is sensitive to both time and temperature. The amount of alpha-crystallin that binds to the membrane increases under acidic pH conditions and upon removal of exposed intrinsic membrane protein domains but is not affected at high ionic strength, suggesting that alpha-crystallin binds to the fiber cell plasma membranes mainly through hydrophobic interactions. The binding capacity and affinity for the reconstituted 3:1 heteromeric complex were measured to be 3. 45 +/- 0.11 ng/microg of membrane and 4.57 +/- 0.50 x 10(-4) microg(-1) of membrane, respectively. The present membrane binding data support the hypothesis that the physical properties of a mixed alpha-crystallin complex may hold particular relevance for the function of alpha-crystallin within the lens.

Membrane lipid alpha-crystallin interaction and membrane Ca2+ -ATPase activities

PURPOSE

To determine the effect of alpha-crystallin binding on lens membrane lipid characteristics and the stability of Ca2+ -ATPase activity when challenged with H2O2 or elevated temperatures.

METHODS

Alpha-Crystallin binding to muscle sarcoplasmic reticulum membranes was quantified using a centrifugation protocol. Alpha-Crystallin binding to lens epithelial lipids was measured by a fluorescence energy transfer technique. Lipid phase transition temperature and lipid order was measured using fluorescence spectroscopy. Ca2+ -ATPase activity was measured using classical biochemical assays.

RESULTS

The main phase transition temperatures of multilamellar vesicles composed of sphingomyelin or lipids extracted from bovine lens were 40 degrees C and 20 degrees C, respectively. In the presence of saturating amounts of alpha-crystallin, the phase transition temperature and lipid order of both sphingomyelin and lens lipid membranes remained almost the same as that without alpha-crystallin. The interaction of alpha-crystallin and lipid is likely to be restricted to the membrane surface. The binding of alpha-crystallin did not influence the oxidative or thermal inactivation of the Ca2+ -ATPase pump.

CONCLUSION

Alpha-Crystallin-lens membrane binding does not protect the Ca2+ -ATPase pump from thermal derangement or oxidation by H2O2.

Influence of cholesterol on the interaction of alpha-crystallin with phospholipids

The influence of cholesterol on the binding of alpha-crystallin to pure phospholipid membranes was studied. The rationale of this investigation stems from two unique aspects of human lens cells: an unusually high level of cholesterol in the membranes and the specific binding of alpha-crystallin to membranes. In the absence of cholesterol, binding of alpha-crystallin liposomes composed of either sphingomyelin, disteroyl-phosphatidylcholine or egg-phosphatidylcholine caused a decrease in the fluorescence intensity and anisotropy of the fluorophore NBD-PE. Since this fluorescence probe resides in the polar headgroup region of the membrane, the observed changes indicated that the binding of alpha-crystallin affected the structure of these membrane regions. The ability of alpha-crystallin to modulate membrane structure suggests yet another potential role for this lens protein. Addition of cholesterol markedly decreased the binding of alpha-crystallin to liposomes composed of either sphingomyelin or disteroylphosphatidylcholine and antagonized the capacity of bound alpha-crystallin to decrease membrane surface order. This antagonism could be explained by the ability of cholesterol to directly decrease the anisotropy of the fluorophore in sphingomyelin membranes unexposed to alpha-crystallin. Thus, with cholesterol present, a further decrease in membrane order upon subsequent binding of alpha-crystallin was less likely. The results obtained with the sphingomyelin liposomes are considered most meaningful, since sphingomyelins are the principal phospholipids in the human lens nuclear membrane and cholesterol preferentially interacts with sphingomyelin. We conclude that cholesterol in lipid membranes can antagonize the binding of alpha-crystallin and thus interfere with the capacity of bound alpha-crystallin to alter membrane order. We suggest that such actions of cholesterol might serve to preserve lens membrane structure in the physiological state where the concentration of soluble alpha-crystallin is great.

Intermediate filament cytoskeletal proteins associated with bovine lens native membrane fractions

PURPOSE

To examine the intermediate filament cytoskeletal proteins associated with native membrane fractions isolated from bovine lenses.

METHODS

Decapsulated bovine lenses were divided into cortex and nucleus. The lens regions were homogenized and separated into water-soluble and water-insoluble fractions by centrifugation. Sedimenting membrane fractions were isolated from the water-insoluble fraction by discontinuous sucrose-density-gradient centrifugation and the non-sedimenting membrane fractions were isolated from the Kbr high-density water-soluble fractions by flotation, during overnight centrifugation. The intermediate filament peptides of the membrane fractions were examined by Western blot analysis, using monoclonal antibodies to filensin, cytoskeletal protein 49 (CP49) and vimentin.

RESULTS

Filensin immunoreactive peptides were found in all membrane fractions of both cortex and nucleus. The parent 115 kDa filensin was found almost exclusively in the urea-soluble protein of cortical membrane fractions, and was the predominant filensin immunoreactive peptide only in the urea-soluble protein of the cortical sedimenting membrane fraction isolated from the 25%/45% sucrose density interface. The predominant filensin immunoreactive peptide of all other samples migrated with a M(r) of 53 kDa. CP49 immunoreactive peptides were found almost exclusively in the urea-soluble protein of all membrane fractions from both the cortex and nucleus. The cortical non-sedimenting membrane fraction and the nuclear membrane fraction of the 25%/45% sucrose density interface were notably deficient in CP49. Vimentin immunoreactive peptides were found in both urea-soluble and urea-insoluble proteins of membrane fractions from the cortex only. Vimentin was particularly enriched in the cortical non-sedimenting membrane fraction. The urea-insoluble filensin immunoreactive peptides were only partially removed by alkali extraction, indicating a very avid association with the membrane. Two dimensional electrophoresis revealed that the urea-soluble protein of the major cortical membrane fraction contained two different filensin-derived 53 kDa fragments.

CONCLUSIONS

The non-sedimenting membrane fraction, which may reflect a distinct domain of the lens plasma membrane, possesses a membrane-associated cytoskeletal composition different from that of the major sedimenting membrane fractions.

Glycation mediated crosslinking between alpha-crystallin and MP26 in intact lens membranes

With advancing age, progressive crosslinking occurs between lens crystallin proteins and other lenticular components. This crosslinking may be involved in the development of senile cataracts. Experiments were conducted to determine whether non-enzymatic glycation could be involved in the crosslinking between lens alpha-crystallin and MP26, an abundant lens fiber cell membrane intrinsic protein. In vitro crosslinking of alpha-crystallin and MP26 of bovine lens membranes was observed in presence of two degradation products of ascorbic acid (ASA), dehydroascorbic acid (DHA) and threose. Alkali-washed bovine lens membranes, isolated after glycation with DHA and threose, contained both alpha-crystallin and MP26, as determined by immunoblot and double immunocytochemical labeling studies. In contrast, membranes incubated without these glycating compounds contained only MP26. SDS-PAGE analysis of [125I] alpha-crystallin incubated with lens membranes in the presence of threose showed a higher amount of radioactivity in high molecular weight aggregates than in the aggregates produced when alpha-crystallin and threose were incubated without membranes. A slot-blot immunoassay of alkali-washed human lens membranes showed a higher amount of covalently bound alpha-crystallin in aged, cataractous or diabetic lens membranes than was present in lens membranes from young normal donors. Based on the in vitro results, we hypothesize that non-enzymatic glycation is one of the vivo mechanisms in the crosslinking of alpha-crystallin to lens membrane proteins, such as MP26. This crosslinking may contribute significantly to the development of age-related and diabetic cataracts.

EM immunolocalization of alpha-crystallins: association with the plasma membrane from normal and cataractous human lenses

PURPOSE

To integrate past biochemical findings with past morphological observations of area insoluble material isolated from cataract and aged normal lenses, by determining the spatial distribution of alpha-crystallins associated with the plasma membrane (PM) of nuclear cataractous and age matched normal human lenses.

METHODS

Lenses were homogenized, pelleted and washed several times in 0.05M Tris-Cl (pH 7.2) containing 100mM KCl, 1 mM MgCl2 and 2mM beta-mercaptoethanol, followed by several washes in 8M urea. Urea insoluble pellets (UIP) were labeled before fixation and embedding with rabbit serum raised against alpha-crystallins, followed by goat anti-rabbit IgG conjugated to 5nm gold. Approximately 300 gold particles associated with the PM were counted, for each lens, on several electron microscopy (EM) micrographs. The number of gold particles/um of PM, number of individual vs clusters of gold particles were determined.

RESULTS

Micrographs from both normal and cataractous human lenses clearly demonstrated the association of alpha-crystallins with the PM. Also apparent was the abundant labeling of the PM for cataractous lenses as compared to normal lenses. Quantification of the gold labeling revealed that not only was there an increase in the amount of labeling/um of PM in cataract lenses, but there was also an increased percentage of gold in clusters. These clusters were not only more numerous in cataractous lenses, but also contained a greater number of gold/cluster.

CONCLUSIONS

These findings provide morphological evidence that the PM in nuclear cataract lenses is associated with large aggregates of alpha-crystallin.

Protein associated with human lens 'native' membrane during aging and cataract formation

Plasma membrane contains extrinsic as well as intrinsic proteins. Changes in the extrinsic proteins of lens membrane during human aging and cataract formation have not been investigated in detail. Unlike previous studies which examined lens membrane after being stripped of extrinsic proteins by treatment with chaotropic agents, we have isolated whole or 'native' lens membrane on a sucrose gradient by ultracentrifugation of the total water-insoluble protein. Essentially all of the water-insoluble protein from young to aged to cataractous human lens appeared membrane associated. In young lens (20-37 years old), most of the membrane banded at the 25/45% sucrose interface fraction. This fraction contained relatively little urea-soluble protein and likely represents fiber-cell plasma membrane with its physiologically associated extrinsic and intrinsic proteins. With aging (62-80 years old), about one-third of the membrane, as judged by the distribution of cholesterol, banded at a much higher density (50/58% sucrose fraction). The higher density was due to a great increase in the membrane's relative protein content (protein/cholesterol). Although this extra protein was composed of both urea-insoluble and -soluble fractions, the urea-soluble protein predominated in all lenses. Cataractous lens differed from aged-clear lens in that much more of the total membrane (70-75%) had shifted to the high density and participated in this massive binding of cytosolic proteins. Although alpha-crystallin was the principal extrinsic-membrane protein in young lens, high molecular weight aggregate of modified (acidic) crystallins accounted for the increased extrinsic protein in aging. The extrinsic proteins bound to both clear-aged and cataractous lens membrane were aggregated. In conclusion, examination of human lens native membrane fractions revealed that the association of crystallins with membrane in both aging and cataracts was much greater than previously recognized and most of this increased protein was non-covalently bound to the membrane. Much more of the lens total membrane from cataractous than clear-aged lens was involved in this massive protein association and the protein bound to cataract membrane appeared more highly aggregated.

High capacity binding of alpha crystallins to various bovine lens membrane preparations

This study examines the high capacity binding of intact and carboxyl-terminal-truncated alpha A(alpha A) crystallin to two types of lens membrane preparations; membrane stripped of extrinsic protein and some lipid by extraction with urea and alkali and unextracted membrane isolated by centrifugation of total water insoluble protein on a sucrose gradient (native membrane). High capacity binding of alpha A crystallin to the urea-treated membrane was seen once the alpha A substrate concentration reached about 1 mg/ml of media. The membrane bound up to one mg of alpha A per mg of intrinsic protein (MP26) at a concentration of 5 mg alpha A/ml media, binding 5 to 10 times greater than that seen by others at saturation of the high affinity but low capacity binding sites. No apparent differences were seen between high capacity binding of carboxyl terminal-truncated alpha A (by trypsin) and intact alpha A, although each crystalline could antagonize binding of the other. However, once membrane bound, neither crystallin appeared to grossly displace the other. Using the carboxyl terminal-truncated alpha crystallin as a model substrate, native membrane was seen to have a higher capacity to bind the truncated alpha crystallin than urea-extracted membrane and binding was better correlated with the preexisting alpha A content of the native membrane than its MP26 content. An artificial native membrane was prepared by prebinding the truncated alpha A to urea-extracted membrane. This preparation bound more intact alpha A than urea-extracted membrane bearing no prebound crystallin. We conclude that lens native membrane possesses a high capacity to bind alpha crystallins and that this binding could be mediated through protein-protein interactions with alpha crystallin bound in situ to the membrane as extrinsic protein.

Selective association of crystallins with lens 'native' membrane during dynamic cataractogenesis

Plasma membrane with its associated extrinsic proteins was isolated from normal and cataractous rat lenses by centrifugation of the total water insoluble fraction from homogenized lenses on a discontinuous sucrose gradient. Membrane, which we call "native" membrane, was recovered mainly from the 25/45% sucrose interface. Development of the experimental U18666A cataract resulted in plasma membrane shifting to higher density (the 50/55% sucrose fraction) and great increases in the urea soluble protein content of the lens. At early stages of cataract development, most of the increased urea soluble protein was membrane associated, presumably as extrinsic protein. With advancing cataract, most of the urea soluble protein appeared in an essentially membrane-free pellet fraction. The urea soluble protein associated with the cataract membrane was shown by combined IEF, SDS-PAGE, Western blotting, amino acid compositional analysis and protein sequence determinations to be mainly composed of modified alpha- and beta-crystallins. Alpha A-crystallin truncated by not more than 27 residues from the carboxyl terminus plus beta b1 crystallin truncated by 49 residues from the amino terminus were conclusively identified. In addition to beta b1, a population of six alpha-crystallin derived polypeptides were specifically enriched in the cataract membrane fraction. Four of these six alpha-crystallins appear to be truncated from their carboxyl terminus, a modification which should have increased their hydrophobicity. The pellet fraction, which accumulated in the lens nucleus as the cataract advanced, was enriched in urea soluble gamma-crystallin derived polypeptides. We suggest that protein insolubilization in this experimental cataract involves the selective and tight association of principally modified alpha-crystallins to the fiber cell plasma membrane.